Terahertz wideband modulator devices using phase change material switchable frequency selective surfaces

Phase change materials (PCMs) such as temperature sensitive materials have received considerable attention in the optics and THz regime because of the reversable switching of their electromagnetic properties. In this paper, wideband switchable absorbers based on vanadium dioxide (VO2) have been designed and simulated at the terahertz (THz) regime. The switching functionality is enabled by the phase change of the VO2 layer from a metal state to an insulator through the temperature transition. The phase change temperature is 68 degrees C. The first designed absorber shows a wideband absorption spectrum (fractional bandwidth of 92.3%) with absorptivity above 90% in the metal state, where conductivity of the VO2 layer is 200000 S m(-1), whereas the absorptivity is less than 3% in the insulator state, when the conductivity of the VO2 reaches 200 S m(-1). An equivalent circuit model-based analysis is presented for metal state. Based on the results, the extinction ration is less than -11 dB with 0.9 modulation depth for a wide frequency range from 2.2 to 5.87 THz. This makes the proposed structure a suitable candidate for broadband modulation at THz frequencies. An ultra-broadband absorption spectrum is achieved in the second design utilizing the stacked structure of two VO2 layers in metal state, where each layer acts as a resonator. This absorber prototype offers more than 90% absorptivity within 1.2-6.1 THz corresponding to a relative bandwidth of 135%.